Multifunction electrode with combined heating and ewod drive functionality

ABSTRACT

An EWOD (or AM-EWOD) device includes a reference electrode and a plurality of array elements, each array element including an array element electrode, and control electronics. In a first mode optimized for EWOD actuation, the control electronics is configured to control a supply of time varying voltages to the array element electrodes and the reference electrode, thereby generating an actuation voltage as a potential difference between voltages at the array element electrodes and the reference electrode. The reference electrode includes a first electrical connection and a second electrical connection. In a second mode, the control electronics further is configured to supply an electrical current flow between the first electrical connection and the second electrical connection to generate resistance heat for controlling temperature of the EWOD device. Control may include sensing a temperature of the EWOD device, and switching between operating in the first or second mode based on the sensed temperature.

TECHNICAL FIELD

The present invention relates to microfluidic devices using theElectro-wetting-On-Dielectric (EWOD) principle. EWOD is a knowntechnique for manipulating droplets of fluid on a hydrophobic surface bymeans of an array of electrodes. The invention further relates tomethods of simultaneously driving such a device and providing control ofthe temperature of the device and its contents by release of heat.

BACKGROUND ART

Electro-wetting on dielectric (EWOD) is a well-known technique formanipulating droplets of fluid by application of an electric field. Itis thus a candidate technology for digital microfluidics forlab-on-a-chip technology. An introduction to the basic principles of thetechnology can be found in “Digital microfluidics: is a truelab-on-a-chip possible?”, R. B. Fair, Microfluid Nanofluid (2007)3:245-281).

FIG. 1 shows a part of a conventional EWOD device in cross section. Thedevice includes a lower substrate 72, the uppermost layer of which isformed from a conductive material which is patterned so that a pluralityof electrodes 38 (e.g., 38A and 38B in FIG. 1) are realized. Theelectrode of a given array element may be termed the element electrode38. A liquid droplet 4, may constitute any polar (or partially polar)material (which is commonly also aqueous and/or ionic), and isconstrained in a plane between the lower substrate 72 and a topsubstrate 36. A suitable gap between the two substrates may be realizedby means of a spacer 32, and a non-polar fluid 34 (for example an oil,for example dodecane or silicone oil or any other alkane or mineral oil)may be used to occupy the volume not occupied by the liquid droplet 4.Alternatively, and optionally, the volume not occupied by the liquiddroplet could be filled with air. An insulator layer 20 disposed uponthe lower substrate 72 separates the conductive electrodes 38A, 38B froma first hydrophobic surface 16 upon which the liquid droplet 4 sits witha contact angle 6 represented by 8. On the top substrate 36 is a secondhydrophobic layer 26 with which the liquid droplet 4 may come intocontact. Interposed between the top substrate 36 and the secondhydrophobic layer 26 is a top substrate electrode 28.

The contact angle θ 6 is defined as shown in FIG. 1, and is determinedby the balancing of the surface tension components between thesolid-liquid (γ_(SL)), liquid-gas (γ_(LG)) and non-polar fluid (γ_(SG))interfaces, and in the case where no voltages are applied satisfiesYoung's law, the equation being given by:

$\begin{matrix}{{\cos \; \theta} = \frac{\gamma_{SG} - \gamma_{SL}}{\gamma_{LG}}} & \left( {{equation}\mspace{14mu} 1} \right)\end{matrix}$

In certain cases, the relative surface tensions of the materialsinvolved (i.e. the values of γ_(SL), γ_(LG) and γ_(SG)) may be such thatthe right hand side of equation (1) is less than −1. This may commonlyoccur in the case in which the non-polar fluid 34 is oil. Under theseconditions, the liquid droplet 4 may lose contact with the hydrophobicsurfaces 16 and 26, and a thin layer of the non-polar fluid 34 (oil) maybe formed between the liquid droplet 4 and the hydrophobic surfaces 16and 26.

In operation, voltages termed the EW drive voltages, (e.g. V_(T), V₀ andV₀₀ in FIG. 1) may be externally applied to different electrodes (e.g.element electrodes 38, 38A and 38B, respectively). The resultingelectrical forces that are set up effectively control the hydrophobicityof the hydrophobic surface 16. By arranging for different EW drivevoltages (e.g. V₀ and V₀₀) to be applied to different element electrodes(e.g. 38A and 38B), the liquid droplet 4 may be moved in the lateralplane between the two substrates 72 and 36.

U.S. Pat. No. 6,565,727 (Shenderov, issued May 20, 2003) discloses apassive matrix EWOD device for moving droplets through an array.

U.S. Pat. No. 6,911,132 (Pamula et al., issued Jun. 28, 2005) disclosesa two dimensional EWOD array to control the position and movement ofdroplets in two dimensions.

U.S. Pat. No. 6,565,727 further discloses methods for other dropletoperations including the splitting and merging of droplets, and themixing together of droplets of different materials.

U.S. Pat. No. 7,163,612 (Sterling et al., issued Jan. 16, 2007)describes how TFT based electronics may be used to control theaddressing of voltage pulses to an EWOD array by using circuitarrangements very similar to those employed in active matrix (AM)display technologies.

The approach of U.S. Pat. No. 7,163,612 may be termed “Active MatrixElectro-wetting on Dielectric” (AM-EWOD). There are several advantagesin using TFT based electronics to control an EWOD array, namely:

-   -   Driver circuits can be integrated onto the AM-EWOD array        substrate.    -   TFT-based electronics are well suited to the AM-EWOD        application. They are cheap to produce so that relatively large        substrate areas can be produced at relatively low cost.    -   TFTs fabricated in standard processes can be designed to operate        at much higher voltages than transistors fabricated in standard        CMOS processes. This is significant since many EWOD technologies        require EWOD actuation voltages in excess of 20V to be applied.

A disadvantage of U.S. Pat. No. 7,163,612 is that it does not discloseany circuit embodiments for realizing the TFT backplane of the AM-EWOD.

EP2404675 (Hadwen et al., published Jan. 11, 2012) describes arrayelement circuits for an AM-EWOD device. Various methods are known forprogramming the array and applying an EWOD actuation voltage to the EWODelement electrode. The voltage write function described includes amemory element of standard means, for example, based on Dynamic RAM(DRAM) or Static RAM (SRAM), and input lines for programming the arrayelement. Whilst EWOD (and AM-EWOD) devices can be operated with eitherDC or AC actuation voltages, in practice there are many reasons forpreferring an AC method of driving, as reviewed in the previously citedreference R. B. Fair, Microfluid Nanofluid (2007) (3:245-281). It may benoted that droplets can be actuated and manipulated for a wide range ofAC driving frequencies ranging typically from a few hertz to severalkHz.

A method for implementing an AC driving method in an AM-EWOD device isto apply a ground potential to the top substrate electrode 28. Toprogram a given element electrode in the array to a non-actuated state,the element electrode is written to a ground potential. To program agiven array element electrode 38 to an actuated state, the elementelectrode potential 38 is written to have a potential that alternatesbetween V_(EW) and −V_(EW). However this method of driving has thesignificant disadvantage that the maximum voltage that must be switchedby the transistors in the circuit in order to drive the elementelectrode 38 is required to be 2V_(EW).

U.S. Pat. No. 8,173,000 (Hadwen et al., issued May 8, 2012) describes anAM-EWOD device with array element circuit and method for writing an ACactuation voltage to the electrode. The AC drive scheme described bythis patent utilizes the application of AC signals to both the elementelectrode 38 and to the top substrate electrode 28 of the device.Therefore, the device is capable of generating an electro-wettingvoltage (voltage between the element electrode and the top substrateelectrode 28) that varies between +V_(EW) and −V_(EW), whilst thetransistors in the array element circuit are only ever required tooperate with a rail-to-rail voltage of V_(EW).

Many applications of EWOD technology require that the temperature ofliquid droplets be controlled and/or varied. Examples include moleculardiagnostics, material synthesis and nucleic acid amplification. A numberof approaches have been taken to providing temperature control in amicrofluidic device. One approach to achieving thermal control is tocontrol the temperature of the entire device and its housing by externalmeans, e.g. a hot plate. This suffers from the disadvantages that therates of temperature change that can be achieved are generally low, andthat a long time is required for the whole arrangement to reach thermalequilibrium. A number of other approaches to address this problem havebeen described.

U.S. Pat. No. 7,815,871 (Pamula et al, issued Oct. 19, 2010) discloses adroplet microactuator system incorporating an EWOD device with one ormore heating zones for temperature control.

U.S. Pat. No. 8,459,295 (Kim et al, issued 11 Jun. 2013) discloses amicrofluidic device for droplet manipulation according to the EWODprinciple, wherein one or more of the electrodes on the bottom substratecomprises a heating element in the form of a patterned electrode.

U.S. Pat. No. 8,339,711 (Hadwen et al, issued Dec. 25, 2012) disclosesan AM-EWOD device, with heater elements realized in the same conductivelayer that is used to control droplet motion.

U.S.20130026040 (Cheng et al, application published Jan. 31, 2013)discloses a microfluidic platform comprising an AM-EWOD device with anactive matrix array of independently addressable heating elements, whichmay be formed in the same or different substrates, above or below adroplet handling area. This arrangement provides for independentactuation and heating of liquid droplets.

Each of these approaches has disadvantages, with many of them involvingmultiple layers of patterned material that must be aligned with oneanother, adding complexity and cost to the manufacturing process. Thisis an important consideration for Lab on a Chip applications,particularly where the chip must be disposable for reasons such asbiological or chemical contamination of the surfaces by the reagents andsamples that are used.

SUMMARY OF INVENTION

An EWOD device is provided having a reference electrode to which atleast two separate electrical connections are made, connection A andconnection B. The device may operate in two modes: an EWOD mode toachieve actuation of droplets by electrowetting, and a heating mode tocontrol the temperature of the droplets in the device. The EWOD mode maybe achieved by supplying one or both of connections A and B with avoltage signal, whilst the electrodes of the lower substrate are alsodriven with suitable voltage signals. The heating mode may be achievedby supplying connections A and B with a voltage signal such that thevoltage signals supplied to connection A and connection B are different.Heat is therefore dissipated in the reference electrode by Jouleheating. The resistance of the reference electrode and /or the voltagesignals supplied to connections A and B may be chosen such thatsimultaneous droplet actuation and Joule heating may occur.

An aspect of the invention, therefore, is an electrowetting ondielectric (EWOD) device. In exemplary embodiments, the EWOD deviceincludes a reference electrode, a plurality of array elements, eacharray element including an array element electrode, and controlelectronics. The control electronics is configured to control a supplyof time varying voltages to the array element electrodes and thereference electrode, thereby generating an actuation voltage as apotential difference between voltages at the array element electrodesand the reference electrode. The reference electrode includes a firstelectrical connection and a second electrical connection, and thecontrol electronics further is configured to supply an electricalcurrent flow between the first electrical connection and the secondelectrical connection to generate resistance heat for controllingtemperature of the EWOD device. The EWOD device may be an active matrixelectrowetting on dielectric (AM-EWOD) device.

The control electronics further may include a switch that is switchablebetween an open position and a closed position. The open positioncorresponds to an EWOD actuation mode in which there is no current flowbetween the first electrical connection and the second electricalconnection to optimize EWOD actuation, and the closed positioncorresponds to a heating mode in which current flows between the firstelectrical connection and the second electrical connection to generatethe resistance heat for controlling temperature of the EWOD device.

Another aspect of the invention is a method of controlling the EWOD (orAM-EWOD) device. In exemplary embodiments, the control method includesthe steps of: operating in a first mode for optimized EWOD actuation bysupplying time varying voltages to the array element electrodes and thereference electrode, thereby generating an actuation voltage as apotential difference between voltages at the array element electrodesand the reference electrode; and operating in a second mode fortemperature control further by supplying an electrical current flowacross the reference electrode to generate resistance heat forcontrolling temperature of the EWOD device. The control method furthermay include sensing a temperature of the EWOD device, and switchingbetween operating in the first mode or the second mode based on thesensed temperature.

In described embodiments of the invention, the EWOD device isimplemented as an AM-EWOD, although the invention is not intended to belimited to active matrix type EWOD devices in the broadest sense.

The advantages of the device are:

-   -   The heater is in close proximity to the droplet on the array,        which allows for more efficient and finer control of its        temperature, and for more rapid changes.    -   The capability of the top substrate to perform a heating        function simultaneously with EWOD actuation means that one or        more droplets can be manipulated to perform functions such as        holding them place, and actively mixing or moving them on the        array without interruption to the heating, as is required by        some systems incorporating multifunction electrodes.    -   The use of a single conductive layer for both EWOD and heating        purposes preserves the simplicity of the design and manufacture        of EWOD devices.

To the accomplishment of the foregoing and related ends, the invention,then, comprises the features hereinafter fully described andparticularly pointed out in the claims. The following description andthe annexed drawings set forth in detail certain illustrativeembodiments of the invention. These embodiments are indicative, however,of but a few of the various ways in which the principles of theinvention may be employed. Other objects, advantages and novel featuresof the invention will become apparent from the following detaileddescription of the invention when considered in conjunction with thedrawings.

BRIEF DESCRIPTION OF DRAWINGS

In the annexed drawings, like references indicate like parts orfeatures:

FIG. 1 shows prior art and is a schematic diagram depicting aconventional EWOD device in cross-section;

FIG. 2 shows an exemplary assay measurement system according to a firstembodiment of the invention;

FIG. 3 shows a schematic of the EWOD device according to an exemplaryfirst embodiment of the invention.

FIG. 4 shows a cross section through some of the array elements of theexemplary AM-EWOD device of FIG. 3, and illustrating the principle bywhich a potential difference is created to produce joule heating in thereference electrode, in accordance with a first and exemplary embodimentof the invention;

FIG. 5 is a schematic diagram depicting an overhead view of the topsubstrate of the exemplary AM-EWOD device of FIG. 4, and an exemplarymethod of making connections so as to allow EWOD actuation and heating;

FIG. 6 shows a circuit representation of the reference electrode and itselectrical connections in FIG. 5 according to a first embodiment,allowing it to be driven to achieve EWOD actuation and joule heating;

FIG. 7 is a graphical representation of the voltages V_(T1) and V_(T2)that are connected to the reference electrode of FIG. 5 and the voltageobserved on the electrode itself. FIG. 7 depicts their change over timeand in EWOD actuation and heating modes, according to an exemplarymethod of driving the reference electrode using the circuit of FIG. 6;

FIG. 8 is a schematic diagram depicting an overhead view of the topsubstrate of the exemplary AM-EWOD device of FIG. 4, and a method ofmaking connections so as to allow EWOD actuation and heating accordingto an exemplary second embodiment;

FIG. 9 is a schematic diagram depicting an overhead view of the topsubstrate of the exemplary AM-EWOD device of FIG. 4, and a method ofmaking connections so as to allow EWOD actuation and heating accordingto an exemplary third embodiment;

FIG. 10 shows a cross section through some of the array elements of anAM-EWOD device according to an exemplary fourth embodiment with analternative design incorporating an in-plane reference electrode on thebottom substrate;

FIG. 11 is a schematic diagram depicting an overhead view of the bottomsubstrate of an AM-EWOD device with an alternative design incorporatinga reference electrode on the bottom substrate, and a method of makingconnections so as to allow EWOD actuation and heating according to anexemplary fourth embodiment;

FIG. 12 shows a circuit representation of the reference electrode andits electrical connections in FIG. 5 according to an exemplary fifthembodiment, allowing it to be driven to achieve EWOD actuation and jouleheating;

FIG. 13 is a graphical representation of the voltages V_(T1) and V_(T2)that are connected to the reference electrode of FIG. 5 and the voltageobserved on the electrode itself. FIG. 13 depicts their change over timeand in EWOD actuation and heating modes, according to an exemplarymethod of driving the reference electrode using the circuit of FIG. 12;

FIG. 14 shows a circuit representation of the reference electrode andits electrical connections in FIG. 5 according to an exemplary sixthembodiment, allowing it to be driven to achieve EWOD actuation and jouleheating;

FIG. 15 is a graphical representation of the voltages V_(T1) and V_(T2)that are connected to the reference electrode of FIG. 5 and the voltageobserved on the electrode itself. FIG. 15 depicts their change over timeand in EWOD actuation and heating modes, according to an exemplarymethod of driving the reference electrode using the circuit of FIG. 14.

DESCRIPTION OF REFERENCE NUMERALS

-   A, B connections to the reference electrode-   4 liquid droplet-   6 contact angle 8-   16 First hydrophobic surface-   20 Insulator layer-   26 Second hydrophobic surface-   28 Reference electrode-   31 In-plane counter electrode-   32 Spacer-   34 Non-polar fluid-   36 Top substrate-   38/38A and 38B Array Element Electrodes-   40 Reader-   41 AM-EWOD device-   42 Electrode array-   43 Control electronics-   45 Non-Transitory computer readable medium-   44 Cartridge-   46/46A and 46B Powered output amplifier-   48 Single-pole, single-throw switch-   50 Resistor-   52 Low resistance zone-   54 High resistance zone-   56 Double pole double throw four-way switch-   72 Substrate-   74 Thin film electronics-   82 Connecting wires-   84 AC voltage supply-   86 DC voltage supply

DETAILED DESCRIPTION OF INVENTION

FIG. 2 shows an exemplary droplet microfluidic handling system accordingto an exemplary first embodiment of the present invention. The system isin two parts including a reader 40 and a cartridge 44.

The cartridge 44 may contain an AM-EWOD device 41 as well as (not shown)fluid input ports into the device, and an electrical connection. Thefluid input ports may perform the function of inputting fluid into theAM-EWOD device 41 and generating droplets 4 within the device, forexample by dispensing from input reservoirs as controlled byelectro-wetting. Optionally, the cartridge 44 may also contain externalheaters and coolers (not shown), which may perform the function ofcontrolling the internal temperature of the cartridge, for example byJoule heating or the Peltier effect. As referenced above, in describedembodiments of the invention, the EWOD device is implemented as anAM-EWOD, although the invention is not intended to be limited to activematrix type EWOD devices in the broadest sense.

The reader 40 may contain control electronics 43 and a non-transitorycomputer readable medium 45 storing application software. Theapplication software may be a computer program containing computer codewhich when executed by a computer is configured to perform some or allof the following functions:

-   -   Define the appropriate timing signals to manipulate liquid        droplets 4 on the AM-EWOD device 41.    -   Interpret input data representative of sensor information        measured by the AM-EWOD device 41, including computing the        locations, sizes, centroids and perimeters of liquid droplets on        the AM-EWOD device 41.    -   Use calculated sensor data to define the appropriate timing        signals to manipulate liquid droplets on the AM-EWOD device 41,        i.e. acting in a feedback mode.    -   Define appropriate signals to the control the temperature of the        AM-EWOD device 41 by modulation of one or more heaters or        coolers (not shown), including those integrated within the        AM-EWOD device 41.    -   A graphical user interface (GUI) whereby the user may program        commands such as droplet operations (e.g. move a droplet), assay        operations (e.g. perform an assay), and which may report the        results of such operations to the user.

The control electronics 43 may supply the required voltage and timingsignals to the cartridge 44 in order to manipulate and sense liquiddroplets 4 on the AM-EWOD device 41. The control electronics 43 may alsosupply the required voltage and timing signals for heating circuits inorder to control the temperature of the droplets 4 in AM-EWOD device 41.

The reader 40 and cartridge 44 may be connected together whilst in use,for example by a cable of connecting wires 82, although various othermethods of making electrical communication may be used as is well known.

FIG. 3 is a schematic diagram depicting an AM-EWOD device 41 that mayform part of the cartridge 44 in accordance with an exemplary embodimentof the invention. The AM-EWOD device 41 has a lower substrate 72 withthin film electronics 74 disposed upon the lower substrate 72. The thinfilm electronics 74 includes at least a portion of the controlelectronics, and are arranged to drive the array element electrodes 38.A plurality of array element electrodes 38 are arranged in an electrodearray 42, having X by Y elements where X and Y may be any integer. Aliquid droplet 4 which may constitute any polar liquid and whichtypically may be aqueous, is enclosed between the lower substrate 72 anda top substrate 36, although it will be appreciated that multiple liquiddroplets 4 can be present.

FIG. 4 is a schematic diagram depicting an AM-EWOD device 41, such asthe device of FIG. 3, in cross-section and including a pair of the arrayelements including a plurality of array element electrodes 38A and 38B.The device configurations of FIGS. 3 and 4 bear similarities to theconventional configuration shown in FIG. 1, with the AM-EWOD device 41of FIGS. 3 and 4 further incorporating the thin-film electronics 74disposed on the lower substrate 72. The uppermost layer of the lowersubstrate 72 (which may be considered a part of the thin filmelectronics layer 74) is patterned so that a plurality of the arrayelement electrodes 38 (e.g., 38A and 38B in FIG. 4) are realized. Thearray element electrodes 38 collectively may be termed the electrodearray 42. The term array element electrode may be taken in what followsto refer both to the electrode 38 associated with a particular arrayelement, and also to the node of an electrical circuit directlyconnected to this electrode 38.

The term reference electrode 28 may be understood in all that follows tomean the most general structure for providing a reference potential toliquid droplet 4. The term reference electrode 28 may thus be consideredto describe a structure including any, or multiple, of a top substrateelectrode 28, an in-plane counter electrode 31 or some other means ofconnecting an electrically conductive structure to the droplet, e.g. acatena. The reference electrode 28 may also be directly in contact withthe liquid droplet 4, or may contact the liquid droplet 4 via aninsulator layer and/or hydrophobic coating layer. The term referenceelectrode 28 also is used to describe the electrical circuit nodecorresponding to the physical reference electrode structure.

FIG. 5 is a schematic diagram depicting the top substrate 36 of theAM-EWOD device 41 of FIG. 3 in an overhead view, according to a first,exemplary embodiment of the invention. The reference electrode 28 may bemade of a layer of an electrically conductive material, such as metal,polysilicon, or conductive oxide materials such as indium tin oxide(ITO). The electrical sheet resistance of the conductive layer maytypically be between 0.5 and 500 Ω/sq. Optionally and preferably,transparent conductive materials (e.g. ITO) may be used to enableoptical measurement techniques such as fluorescence in order tocharacterize the reactions occurring in a droplet 4 inside the AM-EWODdevice 41 in real time. Optionally the reference electrode may bepatterned using well known techniques such as photo-lithography,etching, or other suitable technique. to facilitate local variations inthe sheet resistance of the reference electrode 28. Low resistanceelectrical contacts may be made by standard techniques such assoldering, and allow the reference electrode 28 to be driven withvoltages at connections A and B, these voltages being termed V_(T1) andV_(T2) as shown in FIG. 5.

In general, the control electronics is configured to control a supply oftime varying voltages to the array element electrodes and the referenceelectrode to operate in a first mode optimized for EWOD actuation. Inthis first mode, an actuation voltage is generated as a potentialdifference between voltages at the array element electrodes and thereference electrode. In addition, as referenced above the referenceelectrode includes a first electrical connection A and a secondelectrical connection B. The control electronics further is configuredto operate in a second mode to supply an electrical current flow betweenthe first electrical connection and the second electrical connection. Inthis second mode, which is referred to herein at times as a heatingmode, the additional current flow through the reference electrodebetween connections A and B generates resistance heat for controllingtemperature of the AM-EWOD device.

FIG. 6 shows a circuit representation of the reference electrode 28 ofFIG. 5, the electrical connections A and B, and voltage supplies thatare used to drive the reference electrode for EWOD actuation andheating, according to a first exemplary embodiment. Since the referenceelectrode 28 is a conductor, it can be considered as part of anelectrical circuit. In order to facilitate efficient heating of thereference electrode 28, its resistance should form a significantproportion of the total resistance of the heating circuit. The referenceelectrode is modelled in FIG. 6 as a resistor, with terminal connectionsV_(T1) and V_(T2). In the circuit in FIG. 6, the terminal connection Ais connected to a first voltage supply, such as an AC voltage supply 84,which supplies the voltage used as a reference voltage for EWODactuation and for joule heating during the heating mode. Optionally, asis shown in FIG. 6, an output amplifier 46 may be incorporated betweenthe AC voltage supply 84 and the reference electrode 28 in order tomaintain the signal voltage when a current is drawn, such as when thecircuit is in heating mode. The reference electrode 28 is connected viaterminal B to a switch 48, which is in turn connected to a secondvoltage supply, such as DC voltage supply 86, via a resistor 50. Thedisposition of the different parts of the circuit within the AMEWODdevice 41 are shown by their positions within boxes representing the topsubstrate 36 and the AMEWOD device 41, though some of the elements thatare shown as external to the top substrate 36 could equally beincorporated into its structure.

As has been previously outlined, the circuit is designed to operate intwo modes: a first, EWOD mode optimised for EWOD actuation, and a secondheating mode in which current is allowed to flow through the referenceelectrode 28. In this embodiment the mode is selected by the switch 48that is switchable between an open position corresponding to the firstmode, and a closed position corresponding to the second mode.

In the first EWOD mode, switch 48 is open and the reference electrode isdriven by the AC voltage supply 84, resulting in an AC signal V_(T1) atconnection A, with minimal flow of current. In the second heating mode,switch 48 is closed, making a connection to the DC voltage supply 86 andallowing current to flow through the reference electrode 28 betweenconnection A at voltage V_(T1) and connection B at voltage V_(T2). In anAM-EWOD device 41 using TFT electronics for EWOD drive, typically a DCvoltage between −20 and +20V may be used. Switch 48 may be operatedaccording to a duty cycle, and by varying the proportion of the timespent in heating mode the level of heating and thereby the temperatureof the AM-EWOD device 41 can be controlled. The temperature that isachieved in the AM-EWOD device 41 may be sensed and measured usingstandard means such as thermistors and thermocouples, and may becontrolled by coupling the heat output, for example through the dutycycle of the heating mode, with a closed-loop feedback mechanism usingthe measured temperature as an input, such as aproportional-integral-derivative (PID) controller. The power that isrequired to achieve temperature will be dependent on the design andenvironment of the cartridge 44 in which the AM-EWOD device 41 ismounted. Accordingly, temperature control of the AM-EWOD device may beperformed by sensing a temperature of the AM-EWOD device, and switchingbetween operating in the first mode or the second mode based on thesensed temperature.

It should be noted that during the heating mode, the EWOD referencesignal will continue to be present, allowing EWOD actuation of thedroplet 4. Across the reference electrode 28, the EWOD reference voltageV_(T) that is observed will however be attenuated from the level of thevoltage V_(T1) at connection A, sourced from the AC voltage supply 84,towards the level of the DC voltage V_(T2) that is applied at connectionB. The attenuation that will occur at any respective point on thereference electrode 28 will be in proportion to the resistance of thereference electrode 28 between such respective point and connection A,relative to the resistance of the whole circuit, i.e. between the ACvoltage supply 84 and the DC voltage supply 86 in FIG. 6. Theattenuation effect during heating mode is shown in FIG. 7.

FIG. 7 is a graphical representation of the voltages V_(T1) and V_(T2)observed at connections A and B as labelled in FIG. 6, and in theeffective reference voltage for EWOD actuation V_(T) at a location inthe middle of the reference electrode 28 (not shown), over a shortperiod of time. FIG. 7 displays examples of the voltages V_(T1) andV_(T2) observed at connections A and B and the effective referencevoltage for EWOD actuation in the reference electrode 28, V_(T). Thewaveform for V_(T1) shows how it may be driven by the AC voltage supply84, varying between arbitrary voltages of +V and −V. During EWOD mode,switch 48 is open and both the reference electrode 28 voltage V_(T) andV_(T2) at connection B are driven only by the AC voltage supply and thusthey follow the same waveform. In the case of V_(T2), the waveformduring this period is shown as a dotted line to emphasise that it ispassively driven. When switch 48 is closed, heating mode is entered andconnection B is now connected to, and actively driven by, the DC voltagesupply 86 to produce the voltage now seen as V_(T2). In the heatingmode, V_(T) is driven by both the AC voltage V_(T1) connected atconnection A and the DC voltage V_(T2) connected at connection B. In theexemplary method driving shown in FIG. 7, the DC voltage supply is setto −V, with the result that V_(T2) assumes a voltage close to −V and thewaveform seen at V_(T) is therefore an attenuated version of the ACwaveform superimposed on a baseline of −V. It will be apparent thatother DC voltages including 0V or +V could however be used for the DCvoltage supply 86.

The effect of this attenuation of the AC signal will result in a reducedpotential difference between the reference EWOD voltage V_(T) in thereference electrode 28 and the voltage V₀ on the array elementelectrodes 38 used for EWOD drive. In the case where the frequency ofthe AC waveform is below the characteristic droplet response frequencies(as determined by the droplet conductivity), the effectiveelectrowetting voltage V_(EWOD) is given by the root mean square (rms)value of the voltage difference between the voltage V_(T) on thereference electrode 28, and the voltage V₀ on the array element 38. Theelectrowetting effect and the resulting actuation force that can begenerated depend on V_(EWOD). The attenuation of the reference EWODvoltage V_(T) will therefore result in a reduction in the effectivenessof EWOD actuation, and in parts of the reference electrode 28 close tothe connection B driven at V_(T2), the attenuation may be such thatV_(EWOD) is no longer sufficient to perform operations that require apowerful EWOD driving force, such as splitting a droplet 4 into two.V_(EWOD) may nevertheless be maintained adequately to perform alow-voltage droplet operation. For example, the low-voltage dropletoperation may be to allow the position of a droplet 4 on the electrodearray 42 to be maintained during the heating period, since this does notrequire as strong an EWOD driving force. This is advantageous becausedroplets 4 that are not held may otherwise wander under the influence ofother, weaker forces. The attenuation of the effective reference voltagefor EWOD V_(T) during the heating period will occur to a greater extentacross the reference electrode 28 as the location approaches theconnection to V_(T2). The external resistor 50 may therefore optionallybe incorporated either on or off the top substrate 36 in order to reducethe proportional resistance and voltage drop, so that the EWOD referencevoltage signal for droplet actuation is maintained at an effective levelduring heating.

This embodiment has some particular advantages including the following:

-   -   The reference electrode 28 acting as a heater is in close        proximity to the droplet 4 on the electrode array 42, which        allows for more efficient and potentially finer control of its        temperature, and for more rapid changes in temperature,        particularly as compared to a conventional external heater.    -   The use of a single conductive layer for both EWOD and heating        purposes preserves the simplicity of the design and manufacture        of EWOD devices. In addition, the structure of the reference        electrode 28 as a continuous layer of relatively low resistance        means that the voltage drop between points within the area        contacted by a droplet 4 is small. In other arrangements such as        a meandering electrode pattern, a voltage drop may be observed        between nearby points on the same electrode such that if a        droplet 4 is in contact with both points, current can flow        through the droplet 4 and produce electrolysis, resulting in        damage to the device and in chemical changes in the droplet that        may interfere with its intended behaviour in subsequent        reactions. For such systems, a further dielectric layer covering        the electrode is therefore required, which in turn requires the        voltage for EWOD actuation to be increased to achieve the same        manipulations, which can pose a problem in situations where the        available voltages are limited such as in TFT electronics. By        comparison, in the present invention, the top substrate 36 may        be covered with a hydrophobic coating layer made from a material        such as Cytop, and current flow and electrolysis within the        droplet 4 are not observed.    -   The capability of the top substrate to perform a heating        function simultaneously with EWOD actuation means that one or        more droplets can be manipulated to perform functions such as        holding them place, or actively mixing or moving them on the        electrode array 42, without interruption to the heating, as is        required by some other conventional systems incorporating        multifunction electrodes.

It will be apparent to one skilled in the art that a number of variantsof the EWOD and heating circuit of FIG. 6 may also be realized, withoutsubstantially changing the functionality or principles of operation ofthe circuit. For example, an AC supply voltage is shown driving V_(T1)in the first embodiment described by FIG. 6, since this is aparticularly advantageous mode for effective EWOD actuation in anAM-EWOD device where the TFT electronics limit the maximum voltages thatcan be used. It should also be noted that EWOD actuation may be achievedwith other arrangements, such as a second DC voltage supply 86 connectedto this point, and that the heating mode will be facilitated so long asthere is a difference between the voltages V_(T1) and V_(T2). It willalso be apparent that the supply voltage driving V_(T1) could be drivenby a waveform that is of different frequencies at different times.Furthermore, although an AC voltage supply with a square wave profileand a 50% duty cycle is shown in FIG. 7, it will be apparent that V_(T1)can be driven by waveforms with a duty cycle that is not necessarily 50%or that have a different profile such as a sawtooth or sigmoid wave.

In addition, while the reference electrode 28 is shown as a singleregion of conductive material with one pair of connections A and B atvoltages V_(T1) and V_(T2), in alternative embodiments the referenceelectrode 28 could be divided into two or more regions forming two ormore reference electrodes to be driven separately in order to allowregional control of the temperature, by which different temperatures areattained in different regions. A disadvantage of such embodiments isthat movement or other manipulation of droplets 4 must be carefullychoreographed to avoid the situation in which any droplet 4 spans thegap between two reference electrodes 28 that are locally at differentvoltages, since this creates the opportunity for a current to flow andelectrolysis to occur. This situation can arise in heating mode wherethere are variations either in the resistance of the electrodes orbetween the voltages used to drive the electrodes.

FIG. 8 is a schematic diagram depicting the top substrate 36 of theAM-EWOD device 41 of FIG. 3 in an overhead view according to a secondexemplary embodiment. In the second embodiment, the reference electrode28 of the first embodiment shown in FIG. 5 is modified to incorporate aplurality of first regions of low resistance (high conductance) 52. Thismay be achieved by patterning or otherwise adjusting the thickness ofthe conductive material, such as ITO, from which electrode is formedsuch that the second regions or other parts of the reference electrode28 are of higher resistance, or alternatively by composition of theelectrode from two or more different materials, in which case the lowresistance regions 52 may be made of a material with a higherconductance.

Advantages of this second embodiment include

-   -   i) When connecting the reference electrode 28 to the external        circuitry, a low resistance contact can be more easily formed        with the low resistance regions 52.    -   ii) The low resistance regions running in parallel along the        length of the reference electrode act as an extension of the        driving circuit such that the current flow across the higher        resistance region between them should be more homogenous        compared to the arrangement in FIG. 5.

FIG. 9 is a schematic diagram depicting the top substrate 36 of theAM-EWOD device 41 of FIG. 3 in an overhead view according to a thirdexemplary embodiment. In the third embodiment, the reference electrode28 of the second embodiment shown in FIG. 8 is modified to incorporate aplurality of third regions of high resistance 54 having a higherresistance (lower conductance) as compared to the first or secondregions. This may be achieved by patterning or otherwise adjusting thethickness of the conductive material, such as ITO, from which electrodeis formed, or alternatively by composition of the electrode from two ormore different materials. The advantages of this embodiment include thecapability to produce an uneven distribution of heating across thesurface of the reference electrode, with the high resistance regions 54dissipating proportionately more heat. In a cartridge 44 the surroundingmaterial of the AM-EWOD device 41 may be at a lower temperature than thedevice itself during heating, and the result will be a propensity forthe edges of the AM-EWOD device 41 to lose heat by conduction and forthe temperature to fall off from the centre of the electrode array 42.In the arrangement described in this embodiment, the parts of theAM-EWOD device 41 close to the edges of the reference electrode 28 willbe heated most, which may serve to counteract the conductive coolingeffect and thus produce a more even temperature across the electrodearray 42. It will be clear to a person skilled in the art that otherconfigurations using non-uniform resistance in the reference electrode28 are possible to shape the heating achieved in different ways, such asto create one or more heating and temperature gradients across theelectrode array 42.

FIG. 10 is a schematic diagram depicting an AM-EWOD device 41, such asthe device of FIG. 3, in cross-section and including a pair of the arrayelements 38A and 38B in an alternative exemplary fourth embodiment ofthe invention. FIG. 10 is similar to FIG. 4, but omits the top referenceelectrode 28 in the top substrate 36 and instead incorporates a layer ofconductive material patterned in order to form an in-plane referenceelectrode 31 running between the array elements 38A and 38B on thebottom substrate 72.

FIG. 11 shows an overhead view of bottom substrate 72 of the AM-EWODdevice 41 shown in FIG. 10. The in-plane reference electrode 31 isdepicted in a grid pattern in the spaces between the array elements 38to form an in-plane reference electrode 31 on the bottom substrate 72.

The in-plane reference electrode 31 may be driven by V_(T1) and V_(T2)at connections A and B using a similar circuit to that describedpreviously and depicted in FIG. 6, replacing the reference electrode 28in the top substrate 36 with the in-plane reference electrode 31. In thefourth embodiment depicted in FIG. 10, the in-plane reference electrode31 is made by patterning the same conductive layer as is used for thearray elements 38, which has the disadvantage of reducing the efficacyof a given electrowetting voltage V_(EWOD) for producing EWOD actuationsince there are two passes through the dielectric layer 20. However, asis noted previously, alternative structures are possible which will notbe as affected by this, including an in-plane reference electrode 31formed in a separate layer or specific etching or deposition of thedielectric layer 20 so as to expose the in-plane reference electrode 31and allow it to directly contact the hydrophobic layer 16.

Advantages of this fourth embodiment include simplifying the design ofthe electrical connections in a cartridge 44, and the ability toincorporate alternative additional functionality into the top substrate36.

FIG. 12 shows a circuit representation of the reference electrode 28 ofFIG. 5 and an exemplary mode of forming the electrical connections andvoltages that are used for driving it for EWOD actuation and heating,according to an exemplary fifth embodiment. In this case the AC drivingvoltage supply 84 and DC sink voltage 86 of the first embodiment areconnected to the reference electrode 28 via a double pole double throwfour way switch 56, allowing the sources for the voltages driving V_(T1)and V_(T2) to be switched. The advantages of this embodiment include theability to select which part of the electrode array 42 experiences anattenuated reference voltage V_(T) for EWOD actuation.

FIG. 13 is a graphical representation similar to FIG. 7, displayingexamples of the voltages V_(T1) and V_(T2) observed at connections A andB and the effective reference voltage for EWOD actuation in thereference electrode 28, V_(T), when driven according to the fifthembodiment using the circuit depicted in FIG. 12. As for the firstembodiment shown in FIG. 7, whether the system is in EWOD mode orheating mode is controlled primarily by the state of switch 48. In thisembodiment, however, the state of the double pole double throw switch 56controls which of V_(T1) and V_(T2) is driven by the AC signal and theDC signal. Thus, EWOD mode is selected when the switch 48 is open, inwhich case all of V_(T1), V_(T2) and V_(T) will be driven by whichevervoltage supply is connected to V_(T1) by the double pole double throwswitch 56. As in FIG. 7, the waveform at V_(T2) is shown as a dottedline during EWOD mode to emphasize the fact that it is being passivelydriven by the same voltage supply V_(T2). Heating mode is selected whenthe switch 48 is closed and as for FIG. 7, the waveform of the voltageobserved at V_(T) at a respective location along the reference electrode28 is a combination of the voltages V_(T1) and V_(T2) connected atconnections A and B, resulting in an attenuated version of the signalfrom the AC voltage supply 84. For the purposes of illustration, theeffect on V_(T) of opening switch 48 with the double pole double throwswitch 56 in position 2 is shown in FIG. 13, connecting the referenceelectrode solely to the DC voltage supply 86, though this arrangement isunlikely to be used for AM-EWOD driving in practice.

FIG. 14 shows a circuit representation of the of the reference electrode28 of FIG. 5 and an alternative mode of forming the electricalconnections and voltages that are used to drive it for EWOD actuationand heating, according to an exemplary sixth embodiment. Many elementsof this circuit are the same as that of the first embodiment, portrayedin FIG. 6, with the reference electrode 28 connected at connection A toan AC supply 84 via a first output amplifier 46A. However, in thecircuit of the sixth embodiment, the DC voltage supply 86 of the firstembodiment is replaced with a separate connection from node V_(T2) tothe AC voltage supply 84, via a second output amplifier 46B, which has adifferent gain as compared to the first output amplifier 46A. Forexample, the output amplifiers 46A and 46B may have gains of 1 and 0.7respectively. This enables a potential difference to be achieved acrossthe reference electrode 28 to produce heating, while maintaining areference voltage for EWOD actuation that will be no less than thesignal multiplied by the gain of the second output amplifier 46B. Theresistor 50 is no longer required and can be omitted. If the gain of thesecond output amplifier 46B is variable then temperature control can beachieved not only by the control of the duty cycle of the heating moderelative to the EWOD mode by switch 48, but also by varying the gain andthereby the voltage V_(T2) observed at connection B and ultimately theheating produced. Optionally, if the gain of the second output amplifier46B were variable, then the switch 48 could also be omitted, sinceEWOD-only mode could be achieved by matching the gains and the voltagesV_(T1) and V_(T2) instead of opening the switch 48.

FIG. 15 is a graphical representation of the voltages V_(T1) and V_(T2)observed at connections A and B and of the effective reference voltagefor EWOD actuation in the reference electrode 28, V_(T), over a shortperiod of time, according to the fifth embodiment of the device depictedin FIG. 14. As in FIG. 7, two distinct operating modes are shown, withEWOD mode facilitated by opening switch 48 and heating mode facilitatedby closing the same switch 48. Similarly to the EWOD mode shown in FIG.7, in the sixth embodiment during EWOD mode the reference electrode isconnected only via connection A to the AC voltage signal issuing fromthe output amplifier 46A. V_(T) and V_(T2) are therefore driven to thesame voltage as V_(T1), and once more the waveform at V_(T2) is shownwith a dotted line to emphasize that it is not being driven by a secondconnection. When switch 48 is closed and the heating mode is activated,connection B is connected to the second output amplifier 46B and isshown being actively driven as V_(T2) but with a voltage signal ofreduced amplitude resulting from the lower gain of the output amplifier46B. During this period V_(T) therefore assumes an intermediate voltagebetween the two versions of the AC voltage supply 84.

Advantages of this sixth embodiment include:

-   -   i) The potential for heating with less interruption to EWOD        actuation, since there is the option to provide more continuous        heating with slight reduction in EWOD voltage, rather than brief        periods of more significant reduction.    -   ii) Avoiding dissipating heat in resistor 50 found in other        circuit embodiments.

An aspect of the invention, therefore, is an electrowetting ondielectric (EWOD) device. In exemplary embodiments, the EWOD deviceincludes a reference electrode, a plurality of array elements, eacharray element including an array element electrode, and controlelectronics configured to control a supply of time varying voltages tothe array element electrodes and the reference electrode, therebygenerating an actuation voltage as a potential difference betweenvoltages at the array element electrodes and the reference electrode.The reference electrode includes a first electrical connection and asecond electrical connection, and the control electronics further isconfigured to supply an electrical current flow between the firstelectrical connection and the second electrical connection to generateresistance heat for controlling temperature of the EWOD device.

In an exemplary embodiment of the EWOD device, the control electronicsincludes a first voltage supply for supplying a voltage to the firstelectrical connection, and a second voltage supply for supplying avoltage to the second electrical connection.

In an exemplary embodiment of the EWOD device, the control electronicsfurther includes a switch located between the second voltage supply andthe second electrical connection. The switch is switchable between anopen position and a closed position, the open position corresponding toan EWOD actuation mode in which there is no current flow between thefirst electrical connection and the second electrical connection, andthe closed position corresponding to a heating mode in which currentflows between the first electrical connection and the second electricalconnection to generate the resistance heat for controlling temperaturethe EWOD device.

In an exemplary embodiment of the EWOD device, the control electronicsfurther includes a first amplifier located between the first voltagesupply and the first electrical connection for maintaining the voltagesupply to the first electrical connection.

In an exemplary embodiment of the EWOD device, the first voltage supplyis an alternating current (AC) voltage supply, and the second voltagesupply is a direct current (DC) voltage supply.

In an exemplary embodiment of the EWOD device, the control electronicsfurther includes a resistor between the second voltage supply and thesecond electrical connection, the resistor operating to reduce aproportional resistance of, and voltage drop across, the referenceelectrode.

In an exemplary embodiment of the EWOD device, the control electronicsfurther includes a double pole switch for switching source voltages tothe first electrical connection and the second electrical connection.

In an exemplary embodiment of the EWOD device, the first voltage supplyis an alternating current (AC) voltage supply, and the second voltagesupply is supplied by inputting the first voltage supply to a secondamplifier electrically connected with the second electrical connection,the second amplifier having a different gain from the first amplifier.

In an exemplary embodiment of the EWOD device, the reference electrodehas a plurality of first regions of low resistance having a higherconductance relative to other second regions of the reference electrode.

In an exemplary embodiment of the EWOD device, the reference electrodehas a plurality of third regions of high resistance having a lowerconductance relative to the first and second regions of the referenceelectrode.

In an exemplary embodiment of the EWOD device, the first and/or thirdregions run in parallel along a length of the reference electrode.

In an exemplary embodiment of the EWOD device, the reference electrodecomprises a conductive material formed in plane with the array elementelectrodes.

In an exemplary embodiment of the EWOD device, the EWOD device furtherincludes thin film electronics that includes at least a portion of thecontrol electronics, a substrate upon which the thin film electronics isdisposed, and a non-transitory computer readable medium storing acomputer program that is executed to control the control electronics.

In an exemplary embodiment of the EWOD device, the EWOD device is anactive matrix electrowetting on dielectric (AM-EWOD) device

Another aspect of the invention is a method of controlling anelectrowetting on dielectric (EWOD) device, the EWOD device comprising areference electrode and a plurality of array elements, each arrayelement including an array element electrode. The control methodincludes the steps of: operating in a first mode for optimized EWODactuation by supplying time varying voltages to the array elementelectrodes and the reference electrode, thereby generating an actuationvoltage as a potential difference between voltages at the array elementelectrodes and the reference electrode; and operating in a second modefor temperature control further by supplying an electrical current flowacross the reference electrode to generate resistance heat forcontrolling temperature of the EWOD device.

In an exemplary embodiment of the control method, the control methodfurther includes sensing a temperature of the EWOD device, and switchingbetween operating in the first mode or the second mode based on thesensed temperature.

In an exemplary embodiment of the control method, the control method,further includes using a feedback mechanism for controlling temperatureof the EWOD device.

In an exemplary embodiment of the control method, in the second mode theactuation voltage is attenuated, the method further comprisingmaintaining the actuation voltage in the second mode at a level adequateto perform a low-voltage droplet operation.

In an exemplary embodiment of the control method, the low-voltagedroplet operation comprises maintaining a position of a droplet in theEWOD device.

In an exemplary embodiment of the control method, the referenceelectrode is divided into at least a first region and a second region,the control method further comprising operating in the second mode toattain different temperatures in the first and second regions.

Although the invention has been shown and described with respect to acertain embodiment or embodiments, equivalent alterations andmodifications may occur to others skilled in the art upon the readingand understanding of this specification and the annexed drawings. Inparticular regard to the various functions performed by the abovedescribed elements (components, assemblies, devices, compositions,etc.), the terms (including a reference to a “means”) used to describesuch elements are intended to correspond, unless otherwise indicated, toany element which performs the specified function of the describedelement (i.e., that is functionally equivalent), even though notstructurally equivalent to the disclosed structure which performs thefunction in the herein exemplary embodiment or embodiments of theinvention. In addition, while a particular feature of the invention mayhave been described above with respect to only one or more of severalembodiments, such feature may be combined with one or more otherfeatures of the other embodiments, as may be desired and advantageousfor any given or particular application.

INDUSTRIAL APPLICABILITY

The described embodiments could be used to provide a microfluidic deviceutilising the EWOD principle with a means of controlling the temperatureof fluids on the chip. The EWOD device could form a part of alab-on-a-chip system. Such devices could be used in manipulating,reacting and sensing chemical, biochemical or physiological materials.Applications include healthcare diagnostic testing, material testing,chemical or biochemical material synthesis, proteomics, tools forresearch in life sciences and forensic science.

1. An electrowetting on dielectric (EWOD) device comprising: a referenceelectrode; a plurality of array elements, each array element includingan array element electrode; and control electronics configured tocontrol a supply of time varying voltages to the array elementelectrodes and the reference electrode, thereby generating an actuationvoltage as a potential difference between voltages at the array elementelectrodes and the reference electrode; wherein the reference electrodeincludes a first electrical connection and a second electricalconnection, and the control electronics further is configured to supplyan electrical current flow between the first electrical connection andthe second electrical connection to generate resistance heat forcontrolling temperature of the EWOD device.
 2. The EWOD device of claim1, wherein the control electronics includes a first voltage supply forsupplying a voltage to the first electrical connection, and a secondvoltage supply for supplying a voltage to the second electricalconnection.
 3. The EWOD device of claim 2, wherein the controlelectronics further includes a switch located between the second voltagesupply and the second electrical connection; wherein the switch isswitchable between an open position and a closed position, the openposition corresponding to an EWOD actuation mode in which there is nocurrent flow between the first electrical connection and the secondelectrical connection, and the closed position corresponding to aheating mode in which current flows between the first electricalconnection and the second electrical connection to generate theresistance heat for controlling temperature the EWOD device.
 4. The EWODdevice of claim 3, wherein the control electronics further includes afirst amplifier located between the first voltage supply and the firstelectrical connection for maintaining the voltage supply to the firstelectrical connection.
 5. The EWOD device of claim 2, wherein the firstvoltage supply is an alternating current (AC) voltage supply, and thesecond voltage supply is a direct current (DC) voltage supply.
 6. TheEWOD device of claim 2, wherein the control electronics further includesa resistor between the second voltage supply and the second electricalconnection, the resistor operating to reduce a proportional resistanceof, and voltage drop across, the reference electrode.
 7. The EWOD deviceof claim 2, wherein the control electronics further includes a doublepole switch for switching source voltages to the first electricalconnection and the second electrical connection.
 8. The EWOD device ofclaim 4, wherein the first voltage supply is an alternating current (AC)voltage supply, and the second voltage supply is supplied by inputtingthe first voltage supply to a second amplifier electrically connectedwith the second electrical connection, the second amplifier having adifferent gain from the first amplifier.
 9. The EWOD device of claim 1,wherein the reference electrode has a plurality of first regions of lowresistance having a higher conductance relative to other second regionsof the reference electrode.
 10. The EWOD device of claim 9, wherein thereference electrode has a plurality of third regions of high resistancehaving a lower conductance relative to the first and second regions ofthe reference electrode.
 11. The EWOD device of claim 9, wherein thefirst and/or third regions run in parallel along a length of thereference electrode.
 12. The EWOD device of claim 1, wherein thereference electrode comprises a conductive material formed in plane withthe array element electrodes.
 13. The EWOD device of claim 1, furthercomprising: thin film electronics that includes at least a portion ofthe control electronics; a substrate upon which the thin filmelectronics is disposed; and a non-transitory computer readable mediumstoring a computer program that is executed to control the controlelectronics.
 14. The EWOD device of claim 1, wherein the EWOD device isan active matrix electrowetting on dielectric (AM-EWOD) device
 15. Amethod of controlling an electrowetting on dielectric (EWOD) device, theEWOD device comprising a reference electrode and a plurality of arrayelements, each array element including an array element electrode; thecontrol method comprising the steps of: operating in a first mode foroptimized EWOD actuation by supplying time varying voltages to the arrayelement electrodes and the reference electrode, thereby generating anactuation voltage as a potential difference between voltages at thearray element electrodes and the reference electrode; and operating in asecond mode for temperature control further by supplying an electricalcurrent flow across the reference electrode to generate resistance heatfor controlling temperature of the EWOD device.
 16. The control methodof claim 15, further comprising: sensing a temperature of the EWODdevice; and switching between operating in the first mode or the secondmode based on the sensed temperature.
 17. The control method of claim16, further comprising using a feedback mechanism for controllingtemperature of the EWOD device.
 18. The control method of claim 15,wherein in the second mode the actuation voltage is attenuated, themethod further comprising: maintaining the actuation voltage in thesecond mode at a level adequate to perform a low-voltage dropletoperation.
 19. The control method of claim 18, wherein the low-voltagedroplet operation comprises maintaining a position of a droplet in theEWOD device.
 20. he control method of claim 15, wherein the referenceelectrode is divided into at least a first region and a second region,the control method further comprising operating in the second mode toattain different temperatures in the first and second regions.